Scientist have found the most complex organism to date that can survive direct exposure to space: lichen.

The European Space Agency (ESA), which sponsored the research, says the findings bolster the possibility that life was transferred between planets.

Researchers from Spain flew samples of lichen, which are made of algal cells in a mat of fungus, on the outside of a Russian capsule that spent two weeks in orbit.

The organisms survived the high levels of ultraviolet radiation, as well as the vacuum and extreme temperatures of space.

Dr Rosa de la Torre, from Spain's National Institute for Aerospace Technology in Madrid, says post-flight analysis shows the lichens not only survived, but still had the ability to photosynthesise upon their return.

Images taken by electron microscopes showed no cell damage.

"[The experiment shows] for the first time that complex organisms integrated by the association of seaweed and fungi, are able to resist the conditions of space without showing apparent damage," says Professor Leopoldo Sancho, with Complutense University of Madrid.

Sealed in a capsule

Two species of lichen, Rhizocarpon geographicum and Xanthoria elegans, were sealed in a capsule and launched on Russian Soyuz rocket on 31 May 2005.

Upon reaching orbit, the lid of the container holding the lichen was opened, exposing the samples to the space environment for 14.5 days.

The lid was then closed to protect the samples while the capsule returned to Earth.

"The lichens are probably some of the most resilient organisms that you can find," says astrobiologist Professor Charles Cockell, with the UK's Open University, who is familiar with the Madrid team's work.

Lichens have a mineral coating that apparently shields the organisms from the ultraviolet radiation of space, says Dr Rene Demets, who oversaw the project for the ESA.

On Earth, lichens are typically found on the surfaces of rocks and survive extreme conditions, such as high on mountaintops.

Previous studies have shown that simple organisms such as bacteria can survive in space and possibly even on the surface of Mars. Other organisms, such as plant seeds, have not fared as well.

"They could resist the absolute emptiness and the extreme temperatures, but not the radiation," Sancho says.

Follow-up ground and flight studies are planned for September 2007 to determine how long lichens might survive in space, and if they could survive re-entry forces if, for example, they are transported on a meteorite.